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  <div class="section" id="using-the-global-statement">
<h1>Using the <code class="docutils literal notranslate"><span class="pre">global</span></code> statement<a class="headerlink" href="#using-the-global-statement" title="Permalink to this headline">¶</a></h1>
<p>Global variables are dangerous because they can be simultaneously accessed from multiple sections of a program. This frequently results in bugs. Most bugs involving global variables arise from one function reading and acting on the value of a global variable before another function has the chance to set it to an appropriate value.</p>
<p>Global variables also make code difficult to read, because they force you to search through multiple functions or even modules just to understand all the different locations where the global variable is used and modified.</p>
<div class="section" id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
<p>The code below uses global variables and a function to compute the area and perimeter of a rectangle. As you can see, even with two functions it becomes difficult to keep track of how the global variables are used and modified.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">WIDTH</span> <span class="o">=</span> <span class="mi">0</span> <span class="c1"># global variable</span>
<span class="n">HEIGHT</span> <span class="o">=</span> <span class="mi">0</span> <span class="c1"># global variable</span>

<span class="k">def</span> <span class="nf">area</span><span class="p">(</span><span class="n">w</span><span class="p">,</span> <span class="n">h</span><span class="p">):</span>
    <span class="k">global</span> <span class="n">WIDTH</span> <span class="c1"># global statement</span>
    <span class="k">global</span> <span class="n">HEIGHT</span> <span class="c1"># global statement</span>
    <span class="n">WIDTH</span> <span class="o">=</span> <span class="n">w</span>
    <span class="n">HEIGHT</span> <span class="o">=</span> <span class="n">h</span>
    <span class="k">return</span> <span class="n">WIDTH</span> <span class="o">*</span> <span class="n">HEIGHT</span>

<span class="k">def</span> <span class="nf">perimeter</span><span class="p">(</span><span class="n">w</span><span class="p">,</span> <span class="n">h</span><span class="p">):</span>
    <span class="k">global</span> <span class="n">WIDTH</span> <span class="c1"># global statement</span>
    <span class="k">global</span> <span class="n">HEIGHT</span> <span class="c1"># global statement</span>
    <span class="n">WIDTH</span> <span class="o">=</span> <span class="n">w</span>
    <span class="n">HEIGHT</span> <span class="o">=</span> <span class="n">h</span>
    <span class="k">return</span> <span class="p">((</span><span class="n">WIDTH</span> <span class="o">*</span> <span class="mi">2</span><span class="p">)</span> <span class="o">+</span> <span class="p">(</span><span class="n">HEIGHT</span> <span class="o">*</span> <span class="mi">2</span><span class="p">))</span>

<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;WIDTH:&quot;</span> <span class="p">,</span> <span class="n">WIDTH</span><span class="p">)</span> <span class="c1"># &quot;WIDTH: 0&quot;</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;HEIGHT:&quot;</span> <span class="p">,</span> <span class="n">HEIGHT</span><span class="p">)</span> <span class="c1"># &quot;HEIGHT: 0&quot;</span>

<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;area():&quot;</span> <span class="p">,</span> <span class="n">area</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">))</span> <span class="c1"># &quot;area(): 12&quot;</span>

<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;WIDTH:&quot;</span> <span class="p">,</span> <span class="n">WIDTH</span><span class="p">)</span> <span class="c1"># &quot;WIDTH: 3&quot;</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;HEIGHT:&quot;</span> <span class="p">,</span> <span class="n">HEIGHT</span><span class="p">)</span> <span class="c1"># &quot;HEIGHT: 4&quot;</span>
</pre></div>
</div>
</div>
<div class="section" id="solutions">
<h2>Solutions<a class="headerlink" href="#solutions" title="Permalink to this headline">¶</a></h2>
<div class="section" id="encapsulate-the-global-variables-into-objects">
<h3>Encapsulate the global variables into objects<a class="headerlink" href="#encapsulate-the-global-variables-into-objects" title="Permalink to this headline">¶</a></h3>
<p>One common solution for avoiding global variables is to create a class and store related global variables as members of an instantiated object of that class. This results in more compact and safer code.</p>
<p>In the modified code below, the author eliminates the need for the global variables <code class="docutils literal notranslate"><span class="pre">WIDTH</span></code> and <code class="docutils literal notranslate"><span class="pre">HEIGHT</span></code> by encapsulating this data into a class called <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code>.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">width</span><span class="p">,</span> <span class="n">height</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">width</span> <span class="o">=</span> <span class="n">width</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">height</span> <span class="o">=</span> <span class="n">height</span>
    <span class="k">def</span> <span class="nf">area</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">width</span> <span class="o">*</span> <span class="bp">self</span><span class="o">.</span><span class="n">height</span>
    <span class="k">def</span> <span class="nf">circumference</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="p">((</span><span class="bp">self</span><span class="o">.</span><span class="n">width</span> <span class="o">*</span> <span class="mi">2</span><span class="p">)</span> <span class="o">+</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">height</span> <span class="o">*</span> <span class="mi">2</span><span class="p">))</span>

<span class="n">r</span> <span class="o">=</span> <span class="n">Rectangle</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;area():&quot;</span> <span class="p">,</span> <span class="n">r</span><span class="o">.</span><span class="n">area</span><span class="p">())</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="references">
<h2>References<a class="headerlink" href="#references" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li><p><a class="reference external" href="http://c2.com/cgi/wiki?GlobalVariablesAreBad">Cunningham &amp; Cunningham, Inc. - Global Variables Are Bad</a></p></li>
<li><p>PyLint - W0603, global-statement</p></li>
</ul>
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